Milky Way’s black hole the ultimate particle accelerator

Our galaxy’s supermassive black hole is responsible for the mysterious gamma-ray emission from the galactic centre, a new study suggests. Churning magnetic fields around the monster black hole may act like a giant particle accelerator, leading to high-speed collisions that produce the gamma rays.

Extremely energetic gamma rays, with energies in the tens of tera-electronvolts (1 TeV is 1012 eV) have been detected streaming from our galaxy’s centre recently by ground-based gamma-ray observatories such as the High Energy Stereoscopic System (HESS) near Gambsberg, Namibia.

How such high-energy gamma rays are produced has been a mystery. Some scientists have proposed that it is the result of dark matter particles decaying, but others are not so sure (see Astronomers claim dark matter breakthrough).

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A new analysis suggests that the gamma rays are due to a naturally occurring particle accelerator more powerful than the best atom smashers on Earth.

Magnetic luge

David Ballantyne of the University of Arizona in Tucson, US, led the team that carried out the new study. They made detailed calculations based on the particle accelerator scenario, which was proposed in 2004 in a study led by team member Siming Liu of the Los Alamos National Laboratory in New Mexico.

In this scenario, protons in the vicinity of the supermassive black hole at the galaxy’s centre are constantly buffeted by the roiling magnetic fields there. These magnetic interactions accelerate the protons so much they escape the supermassive black hole’s grip and are flung into surrounding gas clouds.

The magnetic field associated with the black hole extends even farther away, so it continues to kick the protons to even higher speeds as they travel outwards.

Some of the protons could reach energies of 1000 TeV this way, says team member Fulvio Melia of the University of Arizona, in Tucson, US. That is about 100 times higher than the energies protons will reach in the Large Hadron Collider being constructed near Geneva, Switzerland.

The researchers show that this scenario can give the right brightness, spatial distribution, and proportion of different energies of gamma rays to match what is observed at the galactic centre.

Diffuse glow

Melia says the idea that decaying dark matter particles are producing the gamma rays cannot be ruled out, but says “there’s no smoking gun” for that scenario. “It’s much more natural to think of the gamma rays as coming from the black hole,” he says, since the gamma rays are coming from a very compact region centred on the black hole itself.

“It seems like a very plausible scenario,” says gamma-ray researcher David Kieda of the University of Utah in Salt Lake City, US, who was not a member of Ballantyne’s team.

In addition to explaining the bright, compact gamma-ray source around the supermassive black hole, the particle accelerator idea also accounts for a fainter, more spread out gamma-ray glow from the surroundings, he says.

Astronomers say the shape and extent of the glow is different from the expected distribution of dark matter. But the glow would occur naturally in the particle acceleration scenario because some of the protons would make it a considerable distance from the black hole before colliding with gas particles and producing gamma rays, he says.